The present invention relates to a process for the purification of crude chlorophosphazene. More specifically, the present invention relates to the utilization of Bronsted bases in a non-aqueous environment and the subsequent removal of any water produced followed by the removal of the purified cyclic trimer of chlorophosphazene. Alternatively, the water produced by the reaction of acid impurities in crude chlorophosphazene and Bronsted bases can be made to form stable hydrates with drying agents such as magnesium sulfate, calcium sulfate or calcium chloride.
Heretofore, in the production of chlorophosphazenes or phosphonitrilic chlorides, purity of the compounds generally was not a factor since their specific uses such as for flame retardants and thermally resistant resins did not require removal of contaminates. Generally, chlorophosphazenes (NPCl.sub.2).sub.x where x equals 3 through 9 are largely produced by the reaction of phosphorus pentachloride and ammonium chloride which results in compounds, that is, x = 3 - 9, having a high degree of impurities. Although several methods of purification are available, a good degree of purification to date has been difficult, if not impossible, to achieve. Thus, chlorophosphazenes which are contaminated with impurities, upon heating in excess of 250.degree.C will result in gelled polymers which are benzene insoluble.
The general approach of prior art methods of purification has been the separation of isomers of chlorophosphazene, either by utilization of the differences of boiling points of the isomers or by the different reactivities of the isomers with aqueous bases. Specific purification methods have involved the extraction of chlorophosphazene from petroleum ether solutions with sulfuric acid, U.S. Pat. No. 3,008,799; controlled crystallization in a variety of solvents, U.S. Pat. No. 3,378,353; separation of the trimer and tetramer from the produced mixtures through distillation involving a spinning band column, U.S. Pat. No. 3,379,510; contacting molten chlorophosphazenes with an inert solvent vapor so as to selectively vaporize the cyclic trimer polymer, separating a solvent vapor phase laden with trimer and some tetramer from the molten polymer residue, condensing it to form a solution of trimer and tetramer in the solvent and subsequently recovering trimer together with some tetramer from the solution, U.S. Pat. No. 3,677,720; steam distillation of chlorophosphazenes resulting in hydrolysis of x = 4 - 9 and hence separation of the trimer, Chemical Abstracts, Volume 77, Page 540, 159648D ( 1972) and saponification and hydrolysis of chlorophosphazenes by treatment with aqueous sodium or ammonium hydroxide resulting in unreacted trimer and tetramer, U.S. Pat. No. 3,694,171. Considering the last method, that is U.S. Pat. No. 3,694,171, the examples clearly indicate that a high percentage, that is from about 5 to 15 percent of tetramer is obtained with the trimer along with fair amounts of residue and the solvent after the water has reacted with and hydrolyzed the high cyclic compounds to form water soluble compounds which are then decanted. According to the above methods, a final high degree or goal of the ultimate purified product is not stated nor do these methods have the ability to produce water-free chlorophosphazenes.